Mercury is a liquid metal found in natural deposits such as ores containing other elements. Mercury is used in dry-cell batteries, fluorescent light bulbs, switches, and other equipment. The major sources of mercury in drinking water are erosion of natural deposits, discharges from refineries and factors, runoff from landfills, and runoff from croplands. Drinking water with high levels of mercury over a long period of time may result in health problems such as kidney damage. The Environmental Protection Agency in the United States has established so-called maximum contaminant levels (MCL) for chemicals such as mercury. MCLs are set as close to the health goals as possible, considering cost, benefits and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. The EPA has set an enforceable regulation for mercury, (MCL), at 0.002 mg/L or 2 ppb. The World Health Organization (WHO) establishes a maximum level of 6 ppb for mercury(II) in drinking water. When routine monitoring indicates that mercury levels are above the MCL, a water supplier must take steps to reduce the amount of mercury so that is below that level. Water suppliers must notify their customers as soon as practical, but no later than 30 days after the system learns of the violation. Additional actions, such as providing alternative drinking water supplies, may be required to prevent serious risks to public health.
Various neurological effects of mercury exposure have been mainly attributed to the organic form of mercury, predominantly methylmercury (MeHg+), which is known to accumulate in the food chain and cross the blood-brain barrier after human ingestion while such findings have added weight to the severity of organic mercury contamination, the threat of inorganic mercury, namely mercury(II) ions (Hg2+), should not be underestimated. In fact, mercury(II) ions are the primary mercury contamination in the aquatic system and the “precursor” form of methylmercury due to bacteria-assisted biotransformation processes. Furthermore, inorganic mercury is known to be more nephrotoxic than its organic form as it primarily accumulates in the kidney proximal tubule cells. The detection and quantification of mercury(II) ion contamination in water systems are of paramount importance, and could potentially be used to assist prevention of mercury ions from entering the food chain.
Detection of environmental contamination such as trace-level toxic heavy metal ions mostly rely on bulky and costly analytical instruments. Low nanomolar (nM) concentrations of mercury(II) ions have been traditionally detected by using spectroscopic methods, including e.g., atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), and atomic fluorescence spectrometry (AFS). However, these approaches require complex sample preparation procedures, expensive and bulky instruments, and professionally trained personnel running the tests. Therefore, they are not well suited for rapid on-site detection of mercury and may not even be available for use in developing countries. While relatively inexpensive test strips are available for mercury testing, this requires a user to match the color of the reacted test strip to a set control which may produce inaccurate results. Moreover, such test strip solutions are often not able to detect low or trace levels of heavy metals such as mercury. A considerable global need exists for portable, rapid, specific, sensitive and cost-effective detection techniques that can be used in resource-limited and field settings.